This isn't specifically targeted at indie apps but https://medium.com/@sm_app_intel/a-bunch-of-average-app-reve... gives a good overview. The main message is that there's such an immense skew to the top earning apps that the average earning can be above the 90th percentile earning; so if you want to earn average app income you have to be in the top 10% of apps.

I was surprised when the article was written they were estimating $2M a day income for the top earning game app (Clash Of Clans).